The '99 spec 3rd Generation hydraulic lifters, have a large 3mm hole in the top, rather than the small 1mm hole that the old lifters were supplied with. This will increase the oil flow around the cylinder heads.

This redesign alleviates the old problem of the lifter getting blocked and the resulting tick as the lifter can no longer attain its full pressure to work properly. The 3rd gen lifters will stop the lifter tick, that is so common on the 6G72 engine.